As shown in FIG. 6, there has been known conventionally a refrigeration cycle apparatus 100 including: a fluid circuit 102 through which a fluid, such as water and brine used for water heating and space heating, for example, flows; and a refrigerant circuit 101 for heating the fluid in the fluid circuit 102. The refrigerant circuit 101 includes: a compressor 103 for compressing a refrigerant; a radiator 104 for radiating heat from the refrigerant compressed in the compressor 103; an expansion mechanism 105 for expanding the refrigerant from which heat has been radiated in the radiator 104; and an evaporator 106 for evaporating the refrigerant expanded in the expansion mechanism 105. The radiator 104 has a high-temperature side flow passage 104a through which the refrigerant flows and a low-temperature side flow passage 104b through which the fluid in the fluid circuit 102 flows. In this radiator 104, the fluid in the low-temperature side flow passage 104b is heated by the refrigerant in the high-temperature side flow passage 104a. The fluid circuit 102 is provided with a tank 107 for storing the fluid heated in the radiator 104 and with a pump 108.
As the temperature of the fluid in the low-temperature side flow passage 104b increases, the temperature of the refrigerant in the high-temperature side flow passage 104a must be increased accordingly. However, when the temperature of the refrigerant in the high-temperature side flow passage 104a is increased, the high-pressure-side pressure of the refrigerant circuit 101 rises. For example, as shown in FIG. 7, when the refrigerant temperature at the outlet (Point E) of the radiator 104 is increased from T1 to T2 (>T1), the high-pressure-side pressure of the refrigerant circuit 101 rises from P1 to P2. Precisely speaking, since the amount of the refrigerant filled in the apparatus is fixed, the high-pressure-side pressure of the refrigerant circuit 101 rises inevitably as the temperature of the fluid in the low-temperature side flow passage 104b increases. As a result, the input to the compressor 103 increases, thus causing a problem of a decrease in the COP of the refrigeration cycle apparatus 100.
For this reason, it is conceived that the fluid to be flowed in the low-temperature side flow passage 104b of the radiator 104 is cooled before it flows into the low-temperature side flow passage 104b. JP 2002-98429 A proposes, as shown in FIG. 8, that a sub heat exchanger 109 is provided at the outlet side of the evaporator 106 of the refrigerant circuit 101, so that a fluid (water) is cooled by a refrigerant at the outlet side of the evaporator 106 of the refrigerant circuit 101 before the fluid flows into the low-temperature side flow passage 104b of the radiator 104. With this configuration, the temperature of the fluid in the low-temperature side flow passage 104b of the radiator 104 decreases, and thus the increase in the high-pressure-side pressure of the refrigerant circuit 101 is suppressed to some extent. As a result, the decrease in the COP of the refrigeration cycle apparatus 100 is suppressed to some extent.